Polymeric binders for ceramic processing

ABSTRACT

A composition suitable for use as a binder of ceramic materials and a method for preparing a ceramic material which provides relatively greater green ceramic strength is disclosed. The improved binder contains a substantially hydrolyzed copolymer made from monomers having ester or amide functional groups, poly(vinyl amine), poly(vinyl formamide) or a copolymer of vinyl alcohol and vinyl amine. The binder is combined with an aqueous solution containing a ceramic powder to make a slurry, and the slurry is subsequently spray dried, pressed and heated to make a ceramic.

TECHNICAL FIELD OF THE INVENTION

This invention relates to polymeric binders for the preparation ofpressed ceramics.

BACKGROUND OF THE INVENTION

Ceramic materials are commonly prepared by mixing powdered ceramicoxides such as magnesia, alumina, titania and zirconia, in a slurryalong with additives, such as dispersants and binders. The slurry may bespray dried to produce ceramic particles. The particles are pressed intoan aggregate structure, called a "green ceramic," having a desired shapeand subsequently subjected to a severe heat treatment known assintering. The sintering process converts the green ceramic into acohesive "fired ceramic", having a nearly monolithic polycrystallineceramic phase.

The binder serves to hold the ceramic particles of the green ceramic inthe desired shape after pressing. The binder can also providelubrication while the particles are pressed. Preferably, the bindercombusts or vaporizes completely during the sintering process leaving notrace of the binder in the fired ceramic. In performing these functions,binders significantly affect the properties of the fired ceramics whichare ultimately produced.

In commercial practice, poly(vinyl alcohols) are widely used as ceramicbinders. Additionally, poly(ethylene oxide) and ethylene-vinyl acetatecopolymers reportedly have been used as binders for particulatematerial, such as granular silica gel.

Although commercially available binders are satisfactory for manyapplications, a need exists for improved binders which provide stillgreater strength in green ceramic materials. Greater green strengthreduces breakage during handling of the green ceramics and, generally,is associated with higher quality fired ceramics. Preferably, theimproved binders would be cheaper and more versatile than previouslyknown binders.

SUMMARY OF THE INVENTION

The present invention is directed to an improved binder composition forpreparing a ceramic material. The binder may comprise a substantiallyhydrolyzed copolymer of a vinyl ester and an N-vinyl amide.

The ester is of the formula: ##STR1## wherein R₁ is an unsaturatedacyclic hydrocarbon group having about 2 to about 4 carbon atoms and R₂is an alkyl group having 1 to about 4 carbon atoms.

The amide is of the formula: ##STR2## wherein R₃ is hydrogen or an alkylgroup having 1 to about 4 carbon atoms, R₄ is an unsaturated acyclichydrocarbon group having about 2 to about 4 carbon atoms and R₅ ishydrogen or an alkyl group having 1 to about 4 carbon atoms. The bindermay also comprise the homopolymers poly(vinyl amine) or poly(vinylformamide). Similarly, a copolymer of vinyl alcohol and vinyl amine maybe used as a binder.

In another aspect, the invention is directed to a copolymer useful inbinder compositions for preparing processed ceramics that may be ablock, an alternating or a random copolymer. The copolymer is of theformula: ##STR3##

wherein R₂ is an alkyl group having 1 to about 4 carbon atoms, R₃ ishydrogen or an alkyl group having 1 to about 4 carbon atoms, and R₅ ishydrogen or an alkyl group having 1 to about 4 carbon atoms. The sum ofn and q is in the range of about 2 to about 100 mole percent, and thesum of p and r is in the range of about 98 to about 0 mole percent. Theratio r/p is about 0 to about 0.67, and the range of the ratio q/n isabout 0 to about 20. Amine groups may be present in the form of a saltderived from an acid or a mixture of acids. Suitable acids include, butare not limited to, formic acid, acetic acid, hydrochloric acid, andsulfuric acid.

The polymeric binders are used to prepare ceramic materials in a processin which a ceramic powder is mixed with an aqueous solution of thepolymeric binder to produce a slurry which is dried to produceparticles. An aggregate, green ceramic structure formed of particlesproduced from a slurry containing the improved binder exhibitsrelatively greater strength than green ceramic structures provided bypreviously known methods. The particles are compacted and heated toproduce a fired ceramic material.

DESCRIPTION OF PREFERRED ASPECTS OF THE INVENTION

The present invention relates to polymeric binders for preparing ceramicmaterials. The method can be used to produce fired ceramic materialsfrom ceramic powders. Suitable powders include but are not limited to:aluminum oxide, silicon nitride, aluminum nitride, silicon carbide,silicon oxide, magnesium oxide, lead oxide, zirconium oxide, titaniumoxide and neodymium oxide. Aluminum oxide is presently preferred. Thepowder can have a weight-averaged median particle size in the range of afew nanometers to about 1/2 millimeter. Powders having a median size inthe range of about 0.5 to about 10 micrometers are preferred.

In one aspect, the ceramic powder is mixed with an aqueous solutioncontaining a polymer to produce a slurry. Preferably, the solution isprepared using deionized water. The slurry may also contain lubricantsand surfactants, such as dispersants and anti-foaming agents.

The polymer is, for example, synthesized from monomers by free radicalpolymerization in the presence of an initiator, such as a peroxide, aperoxyester or an azo initiator. In the synthesis of a copolymer, one ofthe monomers is an ester of the formula: ##STR4## wherein R₁ is anunsaturated acyclic hydrocarbon group having about 2 to about 4 carbonatoms and R₂ is an alkyl group having 1 to about 4 carbon atoms.Preferably, the ester is vinyl formate, vinyl acetate, vinyl propionate,vinyl butyrate, allyl formate, allyl acetate, allyl propionate, allylbutyrate, isopropenyl formate, isopropenyl acetate, isopropenylpropionate or isopropenyl butyrate. More preferably, the ester is vinylacetate.

The amide is of the formula: ##STR5## wherein R₃ is hydrogen or an alkylgroup having from 1 to about 4 carbon atoms, R₄ is an unsaturatedacyclic hydrocarbon group having about 2 to about 4 carbon atoms, and R₅is hydrogen or an alkyl group having 1 to about 4 carbon , atoms.Examples of suitable amides are N-vinyl-formamide, N-vinyl-acetamide,N-vinyl-propionamide, N-vinyl-butyramide, N-methyl-N-vinyl-formamide,N-methyl-N-vinyl-acetamide, N-methyl-N-vinyl-propionamide,N-methyl-N-vinyl-butyramide, N-ethyl-N-vinyl-formamide,N-ethyl-N-vinyl-acetamide, N-ethyl-N-vinyl-propionamide,N-ethyl-N-vinyl-butyramide, N-propyl-N-vinyl-acetamide, andN-butyl-N-vinyl-acetamide. Preferably, the amide isN-methyl-N-vinyl-acetamide, N-vinyl acetamide or N-vinyl formamide.

The copolymer of the ester and the amide is substantially hydrolyzed.During hydrolysis, ester groups and amide groups present in thecopolymer as pendant substituents are cleaved by water to producependant hydroxyl groups and pendant amino groups, respectively. Thepresence of an acid, such as aqueous hydrochloric acid, catalyzes thehydrolysis reaction. Alternatively, the hydrolysis reaction can becatalyzed by a strong base. Preferably, at least about 60 percent, morepreferably, at least about 80 percent of the total pendant estersubstituents in the copolymer are hydrolyzed. The amount of hydrolyzedpendant amide substituents is in the range of about 5 to about 100 molepercent.

Monomers for homopolymerization include, but are not limited to, N-vinylformamide, which may be hydrolyzed after polymerization to formpoly(vinyl amine). A copolymer of vinyl alcohol and vinyl amine also maybe employed as a binder in the present invention.

The polymer preferably has a weight-average molecular weight in therange of about 2,000 to about 500,000 g/mole, and more preferably in therange of about 2,000 to about 250,000 g/mole. A chain transfer agent,such as thioglycolic acid, can be utilized during polymerization of thecopolymer to control the molecular weight.

Spray drying is an evaporative process in which liquid is removed from aslurry containing a liquid and a substantially non-volatile solid. Theliquid is vaporized by direct contact with a drying medium, usually air,in an extremely short retention time, on the order of about 3 to about30 seconds. The primary controlling factors in a spray drying processare particle size, particle size distribution, particle shape, slurrydensity, slurry viscosity, temperature, residence time, and productmoisture.

The viscosity of the slurry must be suitable for handling andspray-drying. Although spray-drying equipment conditions may be adjustedto handle a variety of viscosities, larger particles will usually resultfrom higher viscosity slurries.

Those of ordinary skill in the art are familiar with the spray-dryingprocess used in the production of ceramic materials and will be able tooptimize the control factors of spray-drying to best advantage.Alternatively, the spray drying process may be replaced by other wellknown drying methods, such as granulation, tape casting and slipcasting.

Spray drying of the slurry produces substantially dry, free-flowingpowder particles which contain the ceramic, the binder and the optionalmaterials described above. The dry particles are granules which aregenerally spheroidal in shape and have an effective diameter of about 50to about 200 micrometers. Typically, about 0.5 percent to about 8percent of the binder, based on the dry weight of the ceramic powder, ispresent in the dry particles.

The dry particles are compacted to produce an aggregate, green ceramicstructure. Preferably, the particles are compacted by pressing in dieshaving an internal volume which approximates the shape desired for thefinal fired ceramic product. Alternatively, the particles are compactedby roll compacting or other well-known compacting methods. The spraydried blend of powder, binder, and optional surfactants and lubricantsis relatively free flowing so that it can enter and closely conform tothe shape of the pressing dies.

Inside the dies, the dry particles are subjected to a pressure which istypically in the range of about 5000 to about 50,000 psi. Pressing theparticles produces an aggregate structure, called a green ceramic, whichretains its shape after removal from the die.

Heating the aggregate structure drives off volatile materials such aswater, and burns off organic materials, such as binders or surfactants.When a sufficiently high temperature is reached, the particles of theaggregate structure begin to fuse, but do not fuse completely, andbecome fastened to one another to produce a relatively strong firedceramic material having essentially the desired shape.

In another aspect, the invention relates to a polymer and a method forpreparing a ceramic material which comprises mixing a ceramic powderwith an aqueous solution containing either a salt of a copolymer havingrepeating units, a homopolymer or a vinyl alcohol-vinyl amine copolymerto produce a slurry. The salt of a copolymer having repeating units canbe prepared by copolymerizing an ester and an amide, followed byhydrolyzing pendant carboxylic acid derivative substituents, asdescribed above. If a copolymer is made, the repeating units arepreferably arranged in random order along a principal polymer chain. Thepolymer is of the formula: ##STR6## wherein R₂ is an alkyl group having1 to about 4 carbon atoms, R₃ is hydrogen or an alkyl group having 1 toabout 4 carbon atoms, and R₅ is hydrogen or an alkyl group having 1 toabout 4 carbon atoms. The sum of n and q is in the range of about 2 toabout 100 mole percent, and the sum of p and r is in the range of about98 to about 0 mole percent. The ratio r/p is about 0 to about 0.67, andthe range of the ratio q/n is about 0 to about 20. Amine groups may bepresent in the form of a salt derived from an acid or a mixture ofacids. Suitable acids include, but are not limited to, formic acid,acetic acid, hydrochloric acid, and sulfuric acid. Minor amounts ofother materials, such as a residue of initiators and chain transferagents, may be present with the polymeric salt in insubstantialquantities.

The slurry is, for example, spray dried to produce substantially dryparticles which include the salt. The particles are preferably pressedto produce an aggregate, green ceramic structure and heated to produce afired ceramic material. Alternatively, the particles can be formed intoan aggregate, green ceramic structure by roll compaction or otherwell-known methods.

It is also recognized that the properties of a ceramic such as, but notlimited to, green density, surface quality or milling characteristics,may be varied as desired by adjusting the ratio of the differentmonomers in a copolymer, the degree of hydrolysis of a copolymer and themolecular weight of the polymer used in the binder composition.

The following examples are presented as a means of further communicatingthe invention, but are not intended to limit the scope of the disclosureor the claims.

EXAMPLE 1

A relatively low molecular weight N-methyl-N-vinylamine/vinyl alcoholcopolymer was prepared by hydrolysis of a copolymer of N-methyl-N-vinylacetamide and vinyl acetate. 46.5 grams of vinyl acetate and 350 gramsof toluene were blended in a reactor and heated to 68° C. under anitrogen gas purge. An azo initiator, 0.2 grams of 2,2'-azobis(2-methylpropanenitrile), was added to the reactor to initiate freeradical polymerization. The nitrogen purge was continued and, after 15minutes, 53.5 grams of N-methyl-N-vinyl acetamide in 49.2 grams oftoluene was slowly added over a period of 2 hours using an additionfunnel. Subsequently, the reactor was maintained at 68° C. with nitrogenpurge for an additional 3 hours, and then cooled.

It was determined gravimetrically that the polymerization conversion atthis point was 22 percent. Another 0.2 grams of the azo initiator wasadded over a period of three hours at 71° C., increasing the conversionto 40 percent. Then 2.0 grams of the azo initiator in a carrier liquidconsisting of 10 grams of toluene, 20 grams of hexane and 20 grams oftetrahydrofuran was added to the reaction mixture over a period of aboutthree hours. The total amount of initiator utilized was 2.4 weightpercent, based on the weight of the monomers.

The polymerized reaction mixture was concentrated under vacuum to 140grams and added to hexane to precipitate and harden the polymerizationproduct. The polymer was then separated from the hexane mixture anddissolved in about 60 milliliters of ethanol.

The polymerization product, a copolymer, was substantially hydrolyzedwith 4 equivalents of aqueous 12 normal hydrochloric acid and isolatedby precipitation. More specifically, the mixture described above wasstirred with 167 milliliters of 12 normal hydrochloric acid and 50milliliters of water. The solution so produced was heated to reflux andabout 20 grams of condensate, which contained some residual toluene, wascollected. The solution was then refluxed at 95° C. for 48 hours.

Isolation of the copolymer product was accomplished by precipitation andfiltration. 55 grams of concentrated sulfuric acid was introduceddropwise into the solution with stirring. The acidified solution wasthen precipitated into 3.5 liters of isopropanol and filtered.

The salt of the hydrolyzed copolymer product was recovered from thefilter as a brown solid product. The product was obtained in 35 percentof overall theoretical yield and exhibited a weight-average molecularweight of about 10,200 g/mole as determined by gel permeationchromatography.

EXAMPLE 2

A N-methylvinylamine/vinyl alcohol copolymer salt synthesized by theprocedure described in Example 1 above, was tested as a binder foralumina particles of the type that are commonly used for producingceramic materials. The polymer had a weight-average molecular weight of10,200 g/mole and was obtained in the form of a salt of a substantiallyhydrolyzed copolymer.

A sample of the polymer was mixed with water to prepare a 13.5 weightpercent solution based on the weight of the sample. 53.89 grams of thesolution were combined with an additional 13.6 grams of water, 0.88grams of a commercially available dispersant, and 175 grams of alumina,available from Alcoa under the tradename Alcoa A152 SG. The dispersantis commercially available from Daishowa Chemicals, Inc. under thetradename Marasperse CBOS-4. The combined ingredients were mixed with apropeller stirrer and an additional 11 grams of water were then added todilute a resulting slurry.

The slurry was milled for 3 hours in a 1 liter jar mill containing about250 grams of milling media. The milled slurry was relatively thick.After adding 78 grams of water to the milled slurry, the viscosity ofthe slurry was measured as 700 centipoise using a Brookfield LVTViscometer.

The milled slurry was spray dried in a Yamato DL-41 laboratory spraydryer. Dryer operating conditions were: 250° C. air inlet temperature,atomizing air setting of 1.2, slurry feed pump setting of 5, and dryingair feed rate of 0.7 cubic meters per minute. A dry powder was producedwhich was recovered, screened and stored overnight in a 20 percentrelative humidity chamber.

The screened powder was pressed into four pellets in a Carver laboratorypress, two at 15,000 pounds per square inch pressing force and two at25,000 pounds per square inch pressing force. The pellets wereapproximately 28.7 millimeters in diameter and 5 to 6 millimeters inheight. The dimensions and weights of the pellets were measured and thepellets were crushed to determine the force required to break them.Diametral compression strength (DCS) for each of the pellets wasdetermined from the breaking force and the pellet dimensions. Theaverage diametral compression strength in megapascals for each set oftwo pellets is presented below in Table 1.

                  TABLE 1                                                         ______________________________________                                        Green Ceramic Prepared by Present Method                                      Pressing Force (psi)                                                                         Strength DCS (MPa)                                             ______________________________________                                        15,000         0.60                                                           25,000         1.05                                                           ______________________________________                                    

EXAMPLE 3

The procedure described above in Example 2 was performed again, exceptthat a conventional binder polyvinyl alcohol was used in place of thehydrochloric acid salt of the hydrolyzed copolymer. The conventionalbinder was combined in an amount corresponding to 5.0 weight percent ofactive binder ingredients, based on the weight of the alumina in theslurry. As in Example 2 above, four green ceramic pellets were preparedutilized two different levels of pressing force. The average diametralcompression strengths in megapascals for pellets formed at each of thepressing force levels are presented below in Table 2.

                  TABLE 2                                                         ______________________________________                                        Green Ceramic Prepared by Conventional Method                                 Pressing Force (psi)                                                                         Strength DCS (MPa)                                             ______________________________________                                        15,000         0.67                                                           25,000         0.84                                                           ______________________________________                                    

Comparison of the data in Table 2 and Table 3 reveals that green ceramicpellets prepared with the salt of the hydrolyzed copolymer exhibitedsignificantly greater diametral compression strength than did greenceramic pellets prepared with the conventional polyvinyl alcohol binder.In both examples, the pellets contained 5.0 weight percent of activebinder ingredients. The weight of chloride present in the salt of thehydrolyzed copolymer of Example 2 was included in the weight of activeingredients. This calculation method is believed to favor theconventional binder. Also, diametral compressive strength comparisons atrelatively greater absolute strength values are believed to be moreaccurate. The greater strengths are calculated from greater requiredbreaking forces, which are relatively more amenable to accuratemeasurement.

EXAMPLE 4

The procedure described in Example 2 was performed on the followingpolymer compositions.

                  TABLE 3                                                         ______________________________________                                                                    MW                                                Sample    Polymer Composition                                                                             (g/mol)                                           ______________________________________                                        A         95%/6% poly(vinyl alcohol/                                                                      100,000                                                     vinyl amine)                                                        B         poly(vinyl amine)  10,400                                           C         poly(vinyl amine)  32,000                                           D         poly(vinyl amine)  40,000                                           E         poly(vinyl amine) 225,000                                           F         poly(vinyl formamide)                                                                           225,000                                           ______________________________________                                    

The average diametral compression strengths in megapascals for pelletsformed at 15,000 psi and 25,000 psi for each polymer composition arepresented below in Table 4.

                  TABLE 4                                                         ______________________________________                                               Strength DCS (Mpa) at                                                                          Strength DCS (Mpa) at                                        Pressing Force 15,000                                                                          Pressing Force 25,000                                 Sample psi              psi                                                   ______________________________________                                        A      --               3.06                                                  B      --               0.248                                                 C      0.03             0.142                                                 D      0.06             0.181                                                 E      --               0.224                                                 F      --               0.07                                                  ______________________________________                                    

The poly(vinyl alcohol/vinyl amine) used in Sample A was obtained fromthe Air Products Company. Poly(vinyl amine) and poly(vinyl formamide)may be synthesized using known procedures.

Although particular aspects have been described and examples presentedfor the purpose of clarity, it is not intended that the invention belimited to the described aspects and examples. The scope of theinvention is intended to be as broad as the claims will allow. Further,although certain theories have been advanced, the success of theinvention does not stand or fall with the theories.

We claim:
 1. A method for preparing a ceramic material, whichcomprises:mixing a ceramic powder with an aqueous solution containing asubstantially hydrolyzed polymer of an ester and an amide to produce aslurry, said ester being of the formula: ##STR7## wherein R₁ is a vinylgroup, and R₂ is an alkyl group having 1 to about 4 carbon atoms, andsaid amide being of the formula: ##STR8## wherein R₃ is hydrogen or analkyl group having 1 to about 4 carbon atoms, R₄ is a vinyl group, andR₅ is hydrogen or an alkyl group having 1 to about 4 carbon atoms saidpolymer having a weight-average molecular weight of about 2000 to about500,000 g/mole; drying the slurry to produce particles which includesaid polymer; compacting the particles to produce an aggregatestructure; and heating the aggregate structure to produce a firedceramic material.
 2. The method of claim 1 wherein the ester is vinylacetate.
 3. The method of claim 1 wherein the amide is N-vinyl acetamideor N-vinyl formamide.
 4. The method of claim 1 wherein the amide isN-methyl-N-vinyl-acetamide.
 5. The method of claim 1 wherein thehydrolyzed polymer contains about 5 to about 90 mole percent pendanthydroxyl substituents and about 5 to about 95 mole percent pendant aminosubstituents.
 6. A method for preparing a ceramic material, whichcomprises:mixing a ceramic powder with an aqueous solution containing apolymer having repeating units to produce a slurry, said polymer beingof the formula: ##STR9## wherein R₂ is an alkyl group having 1 to about4 carbon atoms, R₃ is hydrogen or an alkyl group having 1 to about 4carbon atoms, R₅ is hydrogen or an alkyl group having 1 to about 4carbon atoms, the sum of n and q is in the range of about 2 to about 100mole percent, the sum of p and r is in the range of about 98 to about 0mole percent, the ratio r/p is about 0 to about 0.67, and the range ofthe ratio q/n is about 0 to about 20 said polymer having aweight-average molecular weight of about 2000 to about 500,000 g/mole.drying the slurry-to produce particles which include said polymer;compacting the particles to produce an aggregate structure; and heatingthe aggregate to produce a fired ceramic material.
 7. The method ofclaim 6 wherein said repeating units are present in random order.
 8. Themethod of claim 6 wherein said polymer has a weight-average molecularweight of about 2,000 to about 500,000 g/mole.
 9. A method for preparinga ceramic material which comprises:mixing a ceramic powder with anaqueous solution containing a polymer selected from the group consistingof hydrolyzed poly(vinyl amine), poly(vinyl formamide) and poly(vinylalcohol/vinyl amine) to form a slurry; drying the slurry to produceparticles which include said polymer; compacting the particles toproduce an aggregate structure; and heating the aggregate structure toproduce a fired ceramic material.
 10. The method of claim 6 wherein atleast about 80 percent of the total pendant ester substituents in thepolymer are hydrolyzed.
 11. The method of claim 9 wherein at least about80 percent of the total pendant ester substituents in the copolymer arehydrolyzed.